If the size of a wireless electronic transmitting and/or receiving device ("radio device") is not a concern, then the size of an antenna for the device is irrelevant. However, when size is a consideration, as it is with most radio devices and certainly with hand-held devices, the size of the antenna must be reduced. Generally, the length of an antenna is related to the frequency at which data is communicated. To reduce the size of the antenna, the electronic device may be designed to communicate at higher frequencies (and thus shorter wavelengths). An antenna length of at least one-half the wavelength of the communication frequency is generally used. A one-half wavelength antenna uses two elongated conductors substantially coaxially aligned where each conductor is one-quarter wavelength long. For a radio transmitting device, one conductor transmits the signal (the radiating conductor) while the other conductor acts as a ground (the ground conductor).
To further reduce the length of the antenna, the electronic device may use a ground plane. A ground plane acts as a "mirror" portion of the antenna, effectively doubling the length of an elongated conductor with which it is coupled. Thus, a single elongated one-quarter wavelength conductor, together with a ground plane acting as another one-quarter wavelength conductor, combine to act as a one-half wavelength antenna.
If the radio device is to be portable, it generally must be environmentally secure. Many portable radio devices, such as cellular telephones, have antennas fixed to the exterior of a vehicle in which they are located. The antenna may be mounted to the body of the vehicle; however, holes must be made in the body to (1) mount the antenna and (2) pass wires through to interconnect the antenna and the telephone. These holes compromise the environmental integrity of the vehicle and/or telephone.
To avoid making holes into the vehicle to mount the antenna, several antenna systems capacitively couple the antenna to an interior module within the vehicle which in turn is electrically coupled to the telephone. Most cellular telephones used today operate at very high frequencies. At such high frequencies, a capacitor acts as a short circuit. Therefore, these antennas use the dielectric properties of the glass to which they are affixed to electrically (capacitively) couple to the interior module and thus to the cellular telephone without the need to drill holes through the glass.
Such cellular telephone antennas generally cannot use a ground plane, because both the radiating and ground conductors are capacitively coupled to the interior module. With vehicle mounted antennae, however, size is not necessarily a design constraint, and at the very high cellular telephone frequencies, a one-half wavelength antenna is relatively short.
Most antennas are generally fixed. If the antenna is required to be repeatedly rotated or moved, additional problems arise, particularly those with regard to environmental integrity. Water or other contaminants could penetrate into the device at the point of rotation. If the antenna uses a ground plane, the ground plane must be electrically coupled to the antenna by a mechanical connector. If the antenna is to be rotated, the mechanical means will suffer from wear as the two conductive surfaces rub against each other. Over time, this wear will damage the antenna and/or the mechanical connector, and result in failure of the electrical or mechanical connection therebetween. More reliable rotatable electrical connectors are available; however, these electrical connectors are expensive.
Overall, the inventors are unaware of any inexpensive antenna system which allows the use of a shortened quarter wavelength antenna, rotatable or movable with respect to the electronic device, and which provides environmental integrity between the antenna and the electronic device to which it is coupled.